Sub 60nm etchless MRAM devices by ion beam etching fabricated t-shaped bottom electrode

ABSTRACT

A first conductive layer is patterned and trimmed to form a sub 30 nm conductive via on a first bottom electrode. The conductive via is encapsulated with a first dielectric layer and planarized to expose a top surface of the conductive via. A second conductive layer is deposited over the first dielectric layer and the conductive via. The second conductive layer is patterned to form a sub 60 nm second conductive layer wherein the conductive via and second conductive layer together form a T-shaped second bottom electrode. MTJ stacks are deposited on the T-shaped second bottom electrode and on the first bottom electrode wherein the MTJ stacks are discontinuous. A second dielectric layer is deposited over the MTJ stacks and planarized to expose a top surface of the MTJ stack on the T-shaped second bottom electrode. A top electrode contacts the MTJ stack on the T-shaped second bottom electrode plug.

PRIORITY DATA

The present application is a continuation application of U.S. application Ser. No. 16/452,909, filed Jun. 26, 2019, which is a divisional application of U.S. application Ser. No. 16/008,629, filed Jun. 14, 2018, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This application relates to the general field of magnetic tunneling junctions (MTJ) and, more particularly, to etchless methods for forming sub 60 nm MTJ structures.

BACKGROUND

Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define those millions of MTJ cells in each MRAM device and make them non-interacting to each other, precise patterning steps including reactive ion etching (RIE) are usually involved. During RIE, high energy ions remove materials vertically in those areas not masked by photoresist, separating one MTJ cell from another. However, the high energy ions can also react with the non- removed materials, oxygen, moisture and other chemicals laterally, causing sidewall damage and lowering device performance. To solve this issue, pure physical etching techniques such as ion beam etching (IBE) have been applied to etch the MTJ stack to avoid the damaged MTJ sidewall. However, due to their non-volatile nature, IBE etched conductive materials in the MTJ and bottom electrode can be re-deposited into the tunnel barrier, resulting in shorted devices. A new device structure and associated process flow which can form MTJ patterns with desired sizes without plasma etch is desired.

Several patents teach methods of forming an MTJ without etching, including U.S. Pat. No. 9,029,170 (Li et al) and Patent CN107342331 (Wang et al), but these methods are different from the present disclosure.

SUMMARY

It is an object of the present disclosure to provide a method of forming MTJ structures without chemical damage or re-deposition of metal materials on the MTJ sidewalls.

Another object of the present disclosure is to provide a method of electrically isolatedly forming MTJ patterns on top of a T-shaped bottom electrode without using a plasma etch.

Another object of the present disclosure is to provide a T-shaped bottom electrode and electrically isolatedly forming MTJ patterns on top of the bottom electrode without etching.

In accordance with the objectives of the present disclosure, a method for fabricating a magnetic tunneling junction (MTJ) structure is achieved. A first conductive layer is deposited on a first bottom electrode. The first conductive layer is patterned and trimmed to form a sub 30 nm conductive via on the first bottom electrode. The conductive via is encapsulated with a first dielectric layer. The first dielectric layer is planarized to expose a top surface of the conductive via. A second conductive layer is deposited over the first dielectric layer and the conductive via. The second conductive layer is patterned to form a sub 60 nm second conductive layer wherein the conductive via and second conductive layer together form a T-shaped second bottom electrode. MTJ stacks are deposited on the T-shaped second bottom electrode and on the first bottom electrode wherein the MTJ stacks are discontinuous. A second dielectric layer is deposited over the MTJ stacks and planarized to expose a top surface of the MTJ stack on the T-shaped second bottom electrode. A top electrode layer is deposited on the second dielectric layer and contacting the top surface of the MTJ stack on the T-shaped second bottom electrode plug to complete the MTJ structure.

Also in accordance with the objects of the present disclosure, an improved magnetic tunneling junction (MTJ) is achieved. The MTJ structure comprises a sub-60 nm MTJ device on a T-shaped second bottom electrode, a first bottom electrode underlying the T-shaped second bottom electrode, and a top electrode overlying and contacting the MTJ device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of this description, there is shown:

FIGS. 1 through 9 illustrate in cross-sectional representation steps in a preferred embodiment of the present disclosure.

DETAILED DESCRIPTION

In the present disclosure, it is demonstrated that by using a high angle ion beam etching, we can create a T shaped bottom electrode. Since the bottom portion is only sub 30 nm, much smaller than the top portion of sub 60 nm, the later MTJ deposition cannot form a continuous film along the electrode, but forms separate patterns on top. Using this etchless process, any chemical damage and/or conductive metal re-deposition on the MTJ sidewall are avoided, improving the MRAM device performance.

In a typical MTJ process, the MTJ stack is deposited onto a uniformly sized bottom electrode. Plasma etch is used to transfer the photolithography created photoresist pattern into the MTJ stack. A physical etch such as pure Ar RIE or IBE can avoid chemical damage, but the metal re-deposition in this type of etch can cause electrically shorted devices. However, in the process of the present disclosure, the MTJ stack is deposited onto a T-shaped electrode, so that the patterns are formed without using plasma etch, avoiding these issues.

The preferred embodiment of the present disclosure will be described in more detail with reference to FIGS. 1-9. FIG. 1 illustrates a first bottom electrode layer 12 formed on a semiconductor substrate, not shown. The first bottom electrode 12 is preferably Ta, TaN, Ti or TiN. On top of first bottom electrode or circuit 12, a conductive layer 14 such as Ta, TaN, Ti, TiN W, Cu, Mg, Ru, Cr, Co, Fe, Ni or their alloys is deposited to a thickness h1 of 10-100 nm, and preferably ≥50 nm. A dielectric layer 16 such as SiO2, SiN, SiON, SiC, or SiCN is deposited using chemical vapor deposition (CVD) or spin-coating to a thickness h2 of ≥90 nm.

Next, a photoresist is spin-coated and patterned by photolithography, such as 248 nm photolithography, forming photoresist patterns 18 with size d1 of approximately 70-80 nm and height h3 of ≥200 nm.

Now, the dielectric layer 16 and conductive layer 14 are etched by a fluorine carbon based plasma such as CF₄ or CHF₃ alone, or mixed with Ar and N₂. O₂ can be added to reduce the pillar size further. They can alternatively be patterned by a physical etch such as IBE. Metal layer 14 can also be patterned by a physical etch such as IBE or RIE using pure Ar plasma. Dependent on the thickness of the conductive layer 14, the dielectric layer 16 can be partially consumed. The conductive layer's remaining thickness is still h1 (≥50 nm) with pattern size d2 of 15-70 nm, as shown in FIG. 2.

Next, a high angle IBE trimming 20 is applied to the conductive layer 14. The high angle ranges from 70-90° with respect to the surface's normal line. After IBE trimming, as shown in FIG. 3, the conductive layer pattern size decreases to d3, which can range from 10-30 nm, dependent on the IBE trimming conditions such as RF power (500-1000 W) and time (100-500 sec). Due to the protection of the remaining dielectric layer 16 on top of the conductive layer 14 (FIG. 2), and the extremely low vertical etch rate (≤5 A/sec) of IBE at such a large angle, the remaining conductive layer height h4 is the same as h1, or decreases less than 5 nm after this step. Ex-situ IBE trimming is used when the conductive layer 14 is made of inert metals and in-situ IBE trimming is needed for metals that can be readily oxidized in air. Compared to the immersion 193 nm or EUV photolithography which is widely used to deliver similar results in the integrated circuit (IC) industry, this high angle IBE trimming is a much lower cost method. The remaining dielectric layer 16 and photoresist pattern 18 are consumed during the IBE trimming.

As illustrated in FIG. 4, a dielectric material 22 such as SiO₂, SiN, SiON, SiC, SiCN, or amorphous carbon is deposited to a thickness of ≥100 nm to encapsulate the conductive via 14. The dielectric material 22 may be deposited by physical vapor deposition (PVD), chemical vapor deposition (CVD), or atomic layer deposition (ALO). Whether ex-situ or in-situ PVD/CVD/ALD encapsulation is used is dependent on how sensitive these vias are to the atmosphere. Spin-on or amorphous carbon can also be used if the bottom electrode is made of an inert metal.

Chemical mechanical polishing (CMP) is applied to smooth the surface as well as expose the conductive vias 14 underneath, with remaining via height h5 of ≥45 nm, as shown in FIG. 5. Conductive via 14 forms the bottom pillar portion of the T-shaped bottom electrode. Now, the top portion of the T-shape will be formed.

A metal layer 24 such as Ta, TaN, Ti or TiN W, Cu, Mg, Ru, Cr, Co, Fe, Ni or their alloys is deposited with a thickness h6 of 10-100 nm and preferably ≥50 nm over the via 14 and planarized dielectric layer 22, as shown in FIG. 5. Dielectric layer 26 such as SiO₂, SiN, SiON, SiC, or SiCN with thickness h7 of ≥20 nm is deposited over metal layer 24. Photoresist with a thickness h8 of ≥200 nm is deposited and patterned by 248 nm photolithography to form photoresist mask 28. The dielectric 26 and metal layer 24 are etched by RIE, IBE or their combination to form the pattern size d4 of 50-60 nm, as shown in FIG. 6.

FIG. 6 illustrates the T-shaped bottom electrode 14/24 of the present disclosure. The dielectric encapsulation 22 and remaining hard mask 26 are stripped off to expose the entire T-shaped electrode. A fluorine carbon plasma with high carbon/fluorine ratio such as C₄F₈ or CH₂F₂ can be used to strip off materials like SiO₂, SiN, SiON, SiC, or SiCN, without etching the T-shaped bottom electrode. O₂ plasma can be used to strip off spin-on or CVD deposited carbon encapsulation. Ex-situ stripping is used when the metal vias are made of inert metals, but in-situ stripping is needed for metals that can be readily oxidized in air.

Now, as shown in FIG. 7, MTJ film layers are deposited, typically including a seed layer, a pinned layer, a barrier layer, a free layer, and a cap layer, for example. These layers form the MTJ film stack 30. The MTJ stack 30 can be deposited ex-situ, but preferrably, the MTJ stack is deposited in-situ. After the MTJ stack is deposited, it only covers the top of the T-shaped bottom electrode 14/24 as well as the first bottom electrode 12 on the sides. It should be noted that the MTJ stack is discontinuous because of the undercut structure 24/14.

As a result, the MTJ patterns with size d5 (50-60 nm) are formed without plasma etch and thus, without plasma etch-induced chemical damage and/or conductive metal re-deposition on the MTJ sidewalls. Now, as shown in FIG. 8, dielectric layer 32 is deposited and flattened by CMP, for example, wherein the top MTJ surface is exposed. Finally, the top metal electrode 34 is deposited to form the whole device, also preferably in an in-situ method, as shown in FIG. 9.

In the process of the present disclosure, by decoupling the etch process, we can use a high angle ion beam etching to create a T-shaped bottom electrode to allow for etchless MTJ patterns. The top and pillar T-shaped electrode portions' sizes are sub 60 nm and 30 nm, respectively. After MTJ deposition, the same size of 60 nm MTJ patterns can be electrically isolatedly formed on top of the bottom electrode, without using an etching process. This approach avoids any chemical damage and/or conductive metal re-deposition on the MTJ sidewall, thus improving the MRAM device performance.

FIG. 9 illustrates the completed MTJ structure of the present disclosure. We used high angle IBE trimming to fabricate the T-shaped bottom electrode 14/24 to create MTJ patterns 30 without using a plasma etch. This approach avoids any chemical damage and/or conductive metal re-deposition on the MTJ sidewall, improving the MRAM device performance. Dielectric layer 32 covers the MTJ structures. Top electrode 34 contacted the MTJ structure 30.

The process of the present disclosure will be used for MRAM chips of size smaller than 60 nm as problems associated with chemically damaged sidewalls and re-deposition from the bottom electrode become very severe for these smaller sized MRAM chips.

Although the preferred embodiment of the present disclosure has been illustrated, and that form has been described in detail, it will be readily understood by those skilled in the art that various modifications may be made therein without departing from the spirit of the disclosure or from the scope of the appended claims. 

What is claimed is:
 1. A device comprising: a bottom electrode having a first width in a first direction; a first conductive feature disposed directly on a top surface of the bottom electrode, the first conductive feature having a second width in the first direction that is less than the first width; a second conductive feature disposed directly on the first conductive feature, the second conductive feature having a third width in the first direction that is greater than the second width; a first magnetic tunneling junction (MTJ) structure disposed directly on the second conductive feature; a second MTJ structure disposed directly on the top surface of the bottom electrode; and an encapsulation layer extending continuously from the second MTJ structure to the first conductive feature.
 2. The device of claim 1, wherein the first MTJ structure extends to a first height above the substrate and the second MTJ structure extends to a second height above the substrate that is less than the first height.
 3. The device of claim 1, wherein the first MTJ structure has the same third width in the first direction as the second conductive feature.
 4. The device of claim 1, further comprising a third MTJ structure disposed directly on the top surface of the bottom electrode on a first side of the first conductive feature, and wherein the second MTJ structure is disposed directly on the top surface of the bottom electrode on a second side of the first conductive feature, the second side being opposite the first side.
 5. The device of claim 1, further comprising a top electrode disposed directly on a top surface of the first MTJ structure.
 6. The device of claim 5, wherein the top electrode is disposed directly over the second MTJ structure without interfacing with the second MTJ structure.
 7. The device of claim 6, wherein the encapsulation layer extending continuously from a bottom surface of the top electrode to a top surface of the second MTJ structure, the bottom surface the top electrode facing the bottom electrode and the top surface of the second MTJ structure facing away from the bottom electrode.
 8. The device of claim 1, wherein the first MTJ structure includes multiple layers and the second MTJ structure includes the same multiple layers as the first MTJ structure.
 9. A device comprising: a bottom electrode; a conductive feature disposed on the bottom electrode; a first magnetic tunneling junction (MTJ) structure disposed on the conductive feature, the first MTJ structure being a functional MTJ structure; a second MTJ structure disposed directly on the bottom electrode, the second MTJ structure being a first non-functional MTJ structure; and a dielectric layer extending from the second MTJ structure to the first MTJ structure.
 10. The device of claim 9, wherein the conductive feature includes a lower portion having a first width that interfaces with the bottom electrode and a top portion disposed over the lower portion that has a second width that is different than the first width.
 11. The device of claim 10, wherein the second width is greater than the first width.
 12. The device of claim 9, wherein the conductive feature and the first MTJ structure each have opposing sidewall surfaces, and wherein the dielectric layer physically contacts the opposing sidewall surfaces of the conductive feature and the first MTJ structure.
 13. The device of claim 9, further comprising a top electrode disposed on the first MTJ structure, the top electrode being wider than the first MTJ structure.
 14. The device of claim 9, further comprising a third MTJ structure disposed directly on the bottom electrode.
 15. The device of claim 14, wherein the third MTJ is a second non-functional MTJ structure, and wherein the first, second and third MTJ structures are formed of the same material layers.
 16. A device comprising: a bottom electrode; a T-shaped conductive feature disposed over an interfacing with the bottom electrode; a first functional magnetic tunneling junction (MTJ) disposed over and interfacing with the T-shaped conductive feature; a second MTJ structure disposed directly on the bottom electrode on a first side of the conductive feature, wherein the second MTJ structure is a first non-functional MTJ structure; a third MTJ structure disposed directly on the bottom electrode on a second side of the conductive feature, the second side of the conductive feature being opposite the first side, wherein the third MTJ structure is a first non-functional MTJ structure; and a dielectric layer extending along and interfacing with the second MTJ structure, the bottom electrode, the T-shaped conductive feature and the first MTJ structure.
 17. The device of claim 16, further comprising a top electrode disposed over and interfacing with the first MTJ structure, and wherein the top electrode does not interface with the second and third MTJ structures.
 18. The device of claim 16, wherein the T-shaped conductive feature includes a lower portion having a first width from about 10 nanometers to about 30 nanometers and an upper portion disposed over the lower portion with a second width from about 50 nanometers to about 60 nanometers.
 19. The device of claim 16, wherein each of the first, second and third MTJ structure includes a seed layer, pinned layer, a barrier layer, a free layer and a cap layer.
 20. The device of claim 16, wherein the conductive feature includes a metal material. 